Low-temperature vertical carrier transport in layered structures comprised of Si nanocrystals separated in the growth direction by angstrom-thick SiO2 layers exhibits entirely unexpected, well-defined resonances in conductivity. An unusual alternating current (ac) conductivity dependence on frequency and low magnetic field, negative differential conductivity, reproducible N-shaped switching and self-oscillations were observed consistently. The modeled conductivity mechanism is associated with resonant hole tunneling via quantized valence band states of Si nanocrystals. Tight-binding calculations of the quantum confinement effect for different Si nanocrystal sizes and shapes strongly support the tunneling model.
We have studied carrier transport and lateral electrical properties of nanocrystalline Si layers containing size controlled Si nanocrystals. Using results from direct current (dc) and alternating current (ac) conductivity measurements, the charging of Si nanocrystals and Coulomb blockade effect are discussed.
A quantum structure based on Si/SiO2 and fabricated using standard Si technology has strong potential for applications in non-volatile and scaled dynamic memories. Among standard requirements, such as long retention time and endurance, a structure utilizing resonant tunneling offers lower bias operation and faster write/read cycle. In addition, degradation effects associated with Fowlher-Nordheim (FN) hot electron tunneling can be avoided. Superlattices of nanometer size layers of silicon and silicon dioxide were obtained by sputtering. The size of the silicon nanocrystallites (nc-Si) is fixed by the thickness of the silicon layer which limits the size dispersion. A detailed analysis of the storage of charges in the dots, as a function of the nanocrystals size, is investigated using capacitance methods. Constant voltage and constant capacitance techniques are used to monitor the discharge of the structure. Room temperature non-volatile memory with retention times as long as months is evidenced.
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